1. Field of the Invention
This invention is related generally to single sideband receivers and specifically to single sideband receivers used in systems wherein the channels are tightly packed within an allocated frequency range.
2. Description of the Prior Art
There exists a large variety of techniques for the reception, amplification, and demodulation of communication signals. Single sideband modulation, or SSB, is a type of amplitude modulation, AM, wherein one of the sidebands and virtually all of the carrier are eliminated. Single sideband receivers capable of receiving, amplifying and demodulating a single sideband modulated signal are well known and understood in the prior art. The present invention is for a single sideband receiver intended for use in a communications system having tightly packed channels.
One example of a communications system wherein the channels are tightly packed within an allocated frequency range is a power line communications system. A typical signal for a power line communications system is composed of a data signal occupying a discrete frequency spectrum and a pilot tone. Due to bandwidth limitations the frequency of the pilot tone is adjacent to the frequency spectrum of the data signal. The pilot tone acts as a reference signal carrying phase and amplitude information about the data signal. The pilot tone is therefore instrumental in controlling both the amplification and the demodulation of the received signal.
One prior art technique for receiving and demodulating a single sideband signal provides for filtering a received signal followed by translating the signal to an intermediate frequency signal. The intermediate frequency signal is filtered and amplified. The amplified signal is again filtered to separate the pilot tone from the data signal. The pilot tone is used for automatically controlling the gain of the amplifier and as the input signal to a pilot regenerator. The output of the pilot regenerator is used to demodulate the data signal thus producing an audio output signal.
The major objection to this prior art receiver is the large burden this technique places on the filter located before the amplifier. This filter must distinguish the data signal and pilot tone of one channel from the data signal and pilot tones of immediately adjacent channels. This necessitates the use of an expensive filter having the requisite steep skirts.
A second prior art technique provides for filtering the received signal followed by frequency translation of the filtered signal to an intermediate frequency signal. The intermediate frequency signal is filtered to separate the pilot tone from the data signal. The pilot tone and data signal are then amplified in identical, parallel tracking amplifiers. As in the first prior art technique, the amplified pilot tone is used for automatically controlling the gain of both amplifiers and as the input to a pilot regenerator. The output of the pilot regenerator is used to demodulate the data signal thus producing an audio output signal.
This second prior art technique eliminates the large burden on the filter found in the first prior art technique discussed. By separating the pilot tone from the data signal with filters having a bandwidth less than the total bandwidth, out of channel rejection before amplification is greater. Thus, less expensive filters may be utilized. Although the large burden on the filter found in the first prior art technique has been eliminated a new problem area has been introduced. In the second prior art technique, identical, parallel tracking amplifiers are required so that amplification of the pilot tone, which is the reference signal, will be the same as the amplification of the data signal. Identical, parallel tracking amplifiers with low cross coupling are expensive and difficult to maintain.
The present invention through its use of parallel, intermediate frequency, signal paths and a single gain controlled amplifier eliminates the need for both expensive filters and parallel tracking amplifiers as is required by the prior art techniques discussed above. These, and other advantages, are discussed in detail hereinbelow in the description of the preferred embodiment.